U.S. Pat. No. 3,976,836 (hereinafter referred to as the Wheeler patent), assigned to the same assignee as this invention and hereby incorporated by reference, describes and automatic black level setting circuit that operates in a manner related to the subject invention. The circuit described therein effects, in essence, automatic brightness control for a television receiver CRT by sampling the blackest portion of the transcribed video signal and comparing it to a viewer-established reference level. A comparator circuit develops a DC voltage in response to the difference between the sampled video signal and the reference level. The DC voltage is coupled to a control element (cathode) of the CRT so as to maintain its brightness at the desired level, regardless of fluctuations in or variations between the received video signals. That invention represents a substantial improvement over prior art techniques that established an automatic brightness control (i.e., CRT black level) by sampling, for example, the horizontal sync tips or back porch of the horizontal blanking pulse. Such prior art techniques were undesirably susceptible to aberrations in the level of the sync pulses as well as to station-to-station variations in those levels. Accordingly, in order to establish a black level solely in response to the blackest portion of the video signal, it was necessary to render the comparator immune to, inter alia, the horizontal sync components of the composite video signal. This was effectively accomplished by disabling, in various fashions, the comparator circuit through the use of the approximately 12 microsecond wide horizontal blanking pulses that are developed by the receiver during the horizontal synchronization intervals.
Although the invention disclosed in the Wheeler patent represents a substantial improvement over the then-existing art, it manifests an inherent drawback when receiving particular types of video signals. To wit, when receiving scenes approaching a white field, as are encountered, for example, during the broadcast of a hockey game, in which, by definition, the blackest portion of the video signal may be at a white level, the black level setting circuit operates so as to abnormally reduce the brightness level. It is that drawback of an otherwise salutory circuit to which the subject invention is directed.